doc: announce clarification of implicit filter by port

doc: announce clarification of implicit filter by port

Transfer flow rules may be applied to traffic entering switch from
many sources. There are flow API pattern items which allow to specify
ingress port match criteria explicitly, but it is not documented
if ethdev port used to create flow rule adds any implicit match
criteria and how it coexists with explicit ones.

These aspects should be documented and drivers and applications
which use it in a different way must be fixed.

Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>